1. Field of the Invention
The present invention relates to a hologram reconstructing apparatus configured to reconstruct data recorded on a hologram recoding medium and, more specifically, relates to a hologram reconstructing method of reconstructing a hologram by oversampling a signal obtained by an image sensor that receives a reconstruction signal beam so as to reconstruct data.
2. Description of the Related Art
Recently, rapid development in holographic technologies has been carried out to put holographic memories that have been drawing attention as candidates of large-capacity storage capable of competing against next-generation and next-next-generation optical disks to practical use. Holographic data storage systems for recording and reconstructing large-volume data by employing holographic technologies have been developed.
A holograph recording and reconstructing apparatus (volume hologram memory) records an interference pattern of coherent signal and reference beams in a hologram recording medium (such as photopolymer), generates a reconstruction signal beam by irradiating the hologram recording medium with the same reference beam that was used for recording, and obtains a reconstruction image signal by photoelectrically converting the reconstruction signal beam using an image pickup device, such as a CCD. Since the entire volume of a hologram recording medium is used for recording, with hologram recording, a hologram recording medium is possible to have a significantly greater recording density and recording capacity compared with a known optical disk memory used for two-dimensional information recording.
For such a volume hologram memory, a multiplex recording method is employed to increase recording density by recording a plurality of independent pages at a same area in the hologram recording medium. There are various multiplex recording methods, and some typical methods may include angle multiplexing, polytopic multiplexing, shift multiplexing, phase-code multiplexing, and speckle multiplexing.
FIG. 5 is a block diagram illustrating the structure of a typical hologram recording and reconstructing apparatus employing an angle multiplexing method (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-234179). When recording data in a hologram recording medium 60 using the apparatus illustrated in FIG. 5, while a shutter 2 is closed (during recording, a shutter 5 is always closed), a data page to be recorded is displayed on a spatial modulator (transmissive liquid crystal device) 7 and a spindle motor 10 is rotated to determine the recording site (recording area) in the hologram recording medium 60. Then, after the angle of a movable mirror 9 is determined, the shutter 2 is opened.
Accordingly, a coherent laser beam emitted from a laser beam source 1 enters a beam expander 3 through the shutter 2. In this way, the beam diameter is increased to a diameter that completely covers the modulation area of the spatial modulator 7. Subsequently, the laser beam enters a beam splitter 4 and is split into a signal beam 100 and a reference beam 200. The reference beam 200 is deflected by a movable mirror 9, and is emitted at the hologram recording medium 60 via an optical system for changing the incident angle of the reference beam to the medium (not shown). The incident angle of the reference beam 200 to the hologram recording medium 60 corresponds to the angle of the movable mirror 9.
The signal beam 100 is incident on the spatial modulator 7 via the optical unit 6. Spatial modulation (amplitude modulation) is carried out on the signal beam 100 that passes through the spatial modulator 7 displaying a data page. The spatial modulator 7 includes, for example, a liquid crystal display and generates a spatial modulation pattern, such as that shown in FIG. 6A, by independently changing the transmittance of the pixels. The spatially modulated signal beam 100 is emitted through a signal beam lens 8 so that it overlaps with the reference beam 200 in the hologram recording medium 60. The reference beam 200 and the signal beam 100 that are emitted at the hologram recording medium 60 interfere with each other in the medium 60. A light intensity distribution of the interference pattern that is generated as a result is recorded in the hologram recording medium 60. Then, the shutter 2 is closed.
Subsequently, the next data page to be recorded is displayed on the spatial modulator 7, and, at the same time, the movable mirror 9 rotates slightly so that its angle changes by a predetermined angle. In this state, the shutter 5 is opened to record an interference pattern of another reference beam 200 that has a slightly different incident angle to the hologram recording medium 60 and another signal beam 100 that has been spatially modulated by a data page that is to be recorded next is multiplex- recorded in the same the recording area used for the previous recording. This operation is repeated to multiplex-record a desired number of data pages in one area in the hologram recording medium 60. Subsequently, the hologram recording medium 60 is rotated to move the recording area with respect to the signal beam lens 8, and multiplexed recording is carried out in another recording area in the hologram recording medium 60.
To reconstruct a hologram recorded as described above, the shutter 2 is opened while the shutter 5 is closed so as to irradiate an area in the hologram recording medium 60 where the data page to be reconstructed is recorded with a reference beam 200 (although it is more accurate to refer to this beam as a “reconstructing reference beam,” hereinafter this beam will be simply referred to as a “reference beam”). A hologram reconstruction signal beam 300 generated by the incident reference beam 200 forms an image on an image sensor 12, which is imaging means, via a reconstruction signal beam lens 11. The image sensor 12 is constituted of a CCD or CMOS image sensor that includes many pixels arranged two-dimensionally. By analyzing the intensity of the light incident on each pixel, the modulation pattern, such as that shown in FIG. 6A, is decoded. At this time, by changing the angle of the movable mirror 9, the data pages multiplex-recorded in the same recording area of the hologram recording medium 60 are separated and consecutively reconstructed.
To maximize the data capacity of one page of a hologram so as to maximize the recording capacity of a hologram recording medium, in general, an extremely large the number of pixels (display pixels), i.e., several tens of thousand to several hundreds of thousand pixels, are included in the spatial modulator 7. Therefore, to sufficiently reconstruct data by receiving a reconstruction signal beam once, the number of pixels (light-receiving pixels) included in the image sensor 12, as shown in FIG. 6B, must be at least the same as the number of pixels included in the spatial modulator 7. When the number of pixels included in the image sensor 12 is greater the number of pixels included in the spatial modulator 7, the reconstruction signal beam may be received by the image sensor 12 in a manner such that each pixel represented by the reconstruction signal beam is received by a plurality of pixels on the image sensor 12, in a manner such as that shown in FIG. 6C (i.e., the reconstruction signal beam is oversampled).
FIG. 6C illustrates an example of oversampling when the number of pixels included in the image sensor 12 is four times greater than that of the spatial modulator 7. Compared with a case in which oversampling is not carried out, when oversampling is carried out, the following advantages that contribute to an improvement in the recording density are achieved:
1) the level of degradation in the signal-to-noise ratio (SNR) caused by positional displacement of the reconstruction signal beam in the translational direction and the rotational direction on an image sensor light-receiving plane is low; and
2) the SNR of the light-receiving signal is improved by calculating the brightness of the received light among a plurality of pixels included in the image sensor 12.